Atomic layer deposition system for coating flexible substrates

ABSTRACT

Systems and methods for atomic layer deposition (ALD) on a flexible substrate involve guiding the substrate back and forth between spaced-apart first and second precursor zones, so that the substrate transits through each of the precursor zones multiple times. Systems may include a series of turning guides, such as rollers, spaced apart along the precursor zones for supporting the substrate along an undulating transport path. As the substrate traverses back and forth between precursor zones, it passes through a series of flow-restricting passageways of an isolation zone into which an inert gas is injected to inhibit migration of precursor gases out of the precursor zones. Also disclosed are systems and methods for utilizing more than two precursor chemicals and for recycling precursor gases exhausted from the precursor zones.

RELATED ART

This application claims the benefit under 35 U.S.C. §119(e) from U.S.Provisional Application No. 60/743,786, filed Mar. 26, 2006, which isincorporated herein by reference.

BACKGROUND

The field of this disclosure relates to thin film deposition systems andmethods for coating flexible substrates.

Atomic layer deposition (“ALD”), formerly known as atomic layer epitaxy(“ALE”), is a thin film deposition process that is known for use inmanufacturing electroluminescent (EL) display panels, in semiconductorintegrated circuit manufacturing, and for other purposes. See U.S. Pat.No. 4,058,430 of Suntola et al., and U.S. Patent Application PublicationNos. US 2004/0208994 A1 of Härkönen et al., US 2004/0124131 A1 ofAitchison et al., and US 2005/0011555 A1 of Maula et al., thespecifications of which are all incorporated herein by reference. ALDoffers several benefits over other thin film deposition methods, such asphysical vapor deposition (“PVD”) (e.g., evaporation or sputtering) andchemical vapor deposition (“CVD”), as described in Atomic Layer Epitaxy(T. Suntola and M. Simpson, eds., Blackie and Son Ltd., Glasgow, 1990),incorporated herein by reference.

In contrast to CVD, in which the flows of precursors are static (i.e.,flow rates are steady during processing) and the substrate is exposed tomultiple precursors simultaneously present in the reaction chamber, theprecursor flows in ALD processing are dynamic and sequential, so thatthe substrate is exposed to only one precursor at a time. Successful ALDgrowth has conventionally required the sequential introduction of two ormore different precursor vapors into a reaction space around asubstrate. ALD is usually performed at elevated temperatures and lowpressures. For example, the reaction space may be heated to between 200°C. and 600° C. and operated at a pressure of between 0.1 mbar and 50mbar. In a typical ALD reactor, the reaction space is bounded by areaction chamber sized to accommodate one or more substrates. One ormore precursor material delivery systems (also known as “precursorsources”) are typically provided for feeding precursor materials intothe reaction chamber.

After the substrates are loaded into the reaction chamber and heated toa desired processing temperature, a first precursor vapor is directedover the substrates. Some of the precursor vapor chemisorbs or adsorbson the surface of the substrates to make a monolayer film. In pure ALD,the molecules of precursor vapor will not attach to other like moleculesand the process is therefore self-limiting. Next, the reaction space ispurged to remove excess of the first vapor and any volatile reactionproducts. Purging is typically accomplished by flushing the reactionspace with an inert purge gas that is non-reactive with the firstprecursor. After purging, a second precursor vapor is introduced.Molecules of the second precursor vapor chemisorb or otherwise reactwith the chemisorbed or adsorbed first precursor molecules to form athin film product of the first and second precursors. To complete theALD cycle, the reaction space is again purged with an inert purge gas toremove any excess of the second vapor as well as any volatile reactionproducts. The steps of first precursor pulse, purge, second precursorpulse, and purge are typically repeated hundreds or thousands of timesuntil the desired thickness of the film is achieved.

The required temperatures, pressures, and reaction chamber conditionshave conventionally limited the ALD technique to deposition onsubstrates of relatively small size. For example, known uses of ALDinclude EL display panels and semiconductor wafers.

SUMMARY

In accordance with one embodiment, a system for depositing a thin filmon a flexible substrate includes an isolation zone interposed betweenfirst and second precursor zones. When in use, reactive first and secondprecursor gases are introduced into the respective first and secondprecursor zones, and an inert gas is introduced into the isolation zone.A series of flow-restricting passageways from the isolation zone to thefirst and second precursor zones are spaced apart along the precursorzones. The passageways may include elongated tunnels and/or flexiblewipers to restrict the flow of gases between the isolation zone and theprecursor zones. When in use, a flexible substrate is threaded throughthe passageways so that it traverses back and forth between the firstand second precursor zones multiple times and each time through theisolation zone. A substrate transport mechanism of the system includes aplurality of first turning guides, such as rollers, spaced apart alongthe first precursor zone and a plurality of second turning guides spacedapart along the second precursor zone. At least some of the firstturning guides are adapted to support the substrate during a change in adirection of travel of the substrate toward the second precursor zone,and at least some of the second turning guides are adapted to supportthe substrate during a change in a direction of travel of the substratetoward the first precursor zone. In some embodiments, more than twoprecursor zones are provided, all precursor zones being isolated fromone another. The substrate transport mechanism may include a payoutspool and a take-up spool for roll-to-roll processing of the substrate.

A method of thin film deposition, according to one embodiment, includesintroducing an inert gas into an isolation zone that is interposedbetween first and second precursor zones, introducing first and secondprecursor gases into the respective first and second precursor zones,and then guiding a flexible substrate back and forth between the firstand second precursor zones and through a series of flow-restrictingpassageways of the isolation zone, so that the substrate transitsthrough the first and second precursor zones multiple times. The methodfurther includes generating pressure differentials between the isolationzone and the first precursor zone and between the isolation zone and thesecond precursor zone, the pressure differentials being sufficient toinhibit migration of the first and second precursor gases out of therespective first and second precursor zones and mixing of the first andsecond precursor gases within one of the zones, thereby essentiallypreventing reactions within the zones between nonadsorbed amounts of thefirst and second precursor gases. The pressure differential may beachieved, for example, by differential injection of gases into thevarious zones or by differential pumping or throttling of exhaust gasesfrom the various zones. In some embodiments, an inert gas is injectedinto some or all of the passageways. As the substrate transits throughthe first precursor zone a monolayer of the first precursor gas isadsorbed to the surface of the substrate, and on a subsequent transit ofthe substrate through the second precursor zone the second precursor gasreacts with the adsorbed first precursor at the surface of thesubstrate, to thereby deposit a thin film on the substrate. Many layersof material may be deposited by guiding the substrate along a serpentinepath that traverses between the first and second precursor zones manytimes.

In some embodiments of the method and system, the substrate istransported through three or more precursor zones, all isolated from oneanother by the isolation zone. One or more of the turning guides,precursors, precursor zones, isolation fluid, or isolation zone may beheated.

In some embodiments, the isolation and precursor zones may be operatedat approximately atmospheric pressures, while in others the pressure mayrange from relatively low vacuum pressures (e.g. 1 millitorr) topositive pressures of 500 to 1500 Torr (approx. 1-2 atmospheres).

In some embodiments of the method and system, the flexible substrate maybe advanced continuously along a serpentine path in a first direction tocomplete a first pass, and subsequently rewound along the serpentinepath in a second direction opposite the first direction to complete asecond pass.

Embodiments of the method may also include the steps of switchingprecursors during or between passes, introducing dopants into one ormore precursor zones, and/or introducing a radical into one or more ofthe precursor zones. A length or duration of some of the transitsthrough the precursor zones may be adjusted, in some embodiments, bymovably mounted turning guides or zone dividers.

Systems and methods for trapping exhaust precursor gases for disposal,recycling, or reclaim are also disclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view illustrating a system andmethod for ALD on a flexible substrate, in accordance with a firstembodiment;

FIG. 2 is a schematic cross-sectional view illustrating a system andmethod utilizing ALD for coating layers of different materials onto aflexible substrate, in accordance with a second embodiment;

FIG. 3 is a schematic cross-sectional view illustrating a system andmethod for ALD in which a substrate is moved through a linearmulti-stage ALD reactor, in accordance with a third embodiment; and

FIG. 4 is a schematic cross-sectional view illustrating a system andmethod for ALD on a flexible substrate according to a fourth embodiment,including a precursor recovery and recycling system.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In accordance with embodiments described herein, a flexible substrate,such as a plastic or metallic web or filament, for example, is threadedbetween adjacent zones each having a different precursor chemical orisolation fluid present therein. As the substrate is advanced, eachsegment of the substrate is preferably resident in the precursor zoneslong enough to accomplish the requisite adsorption and reaction ofprecursor chemicals on the substrate surface. An isolation zoneinterposed between the precursor zones prevents mixing of the differentprecursor gases. The substrate is moved through the zones to achieve athin film coating consistent with the coatings deposited by conventionalALD processes. In addition to enabling the deposition of a highlyconformal thin film coating on web materials and on other flexibleelongate substrates, systems and methods according to the embodimentsdescribed herein may avoid the need to deliver into a common reactionchamber a sequence of precursor and purge gas pulses in alternatingsuccession, as is done in a conventional traveling wave-type ALDreactor.

Among other possible benefits, certain systems and methods disclosedherein may facilitate the deposition of barrier layers and transparentconductors on flexible substrates, such as on plastic substrates fororganic light emitting diode (OLED) displays, and the deposition ofconformal coatings on very large substrates. Many additional advantagesand uses of the systems and methods will become apparent from thefollowing detailed description, which proceeds with reference to theaccompanying drawings.

FIG. 1 illustrates a schematic cross-section view of a system 10according to a first embodiment for the deposition of a thin-filmcoating onto a flexible substrate 12 (shown in profile in FIG. 1), suchas a web of plastic film or metal foil, for example. With reference toFIG. 1, system 10 includes first and second precursor zones 14 and 16,respectively, separated by an intermediate isolation zone 20 in which aninert fluid is present. The inert fluid may comprise an inert liquid,but more preferably consists essentially of an inert gas, such asnitrogen (N₂). When in use, reactive first and second precursor gases(Precursor 1 and Precursor 2) are introduced into the respective firstand second precursor zones 14, 16 from first and second precursordelivery systems 24, 26. Precursor delivery systems 24, 26 may includeprecursor source containers (not shown) located outside or withinprecursor zones 14, 16. Additionally or alternatively, precursordelivery systems 24, 26 may include piping, pumps, valves, tanks, andother associated equipment for supplying precursor gases into precursorzones 14, 16. An inert gas delivery system 28 is similarly included forinjecting inert gas into isolation zone 20.

In the embodiment shown, precursor zones 14, 16 and isolation zone 20are defined and bordered by an outer reaction chamber housing or vessel30, divided by first and second dividers 34, 36 into three sub-chambers,namely, a first precursor chamber 44, a second precursor chamber 46 andan inert gas chamber 50. Vessel 30 may comprise a pressure vessel orvacuum vessel substantially isolating the process space from theexternal environment. In other embodiments, the vessel 30 may haveentrance and exit passageways for interfacing with other process modulesor equipment, as described below with reference to FIG. 4. A series offirst passageways 54 through first divider 34 are spaced apart along ageneral direction of travel of substrate 12, and a corresponding seriesof second passageways 56 are provided through second divider 36. Thepassageways 54, 56 are arranged and configured for substrate 12 to bethreaded therethrough back and forth between first and second precursorzones 14, 16 multiple times, and each time through isolation zone 20.For a web substrate, passageways 54, 56 preferably comprise slits havinga width (exaggerated in FIG. 1) that is slightly greater than thethickness of substrate 12 and a length (not shown) extending into theplane of FIG. 1 (i.e., normal to the page) and that is slightly greaterthan a width of the substrate. Isolation zone 20 is, thus, preferablyseparated (albeit imperfectly) from the first precursor zone 14 by firstdivider 34 and from second precursor zone 16 by second divider 36.

To substantially prevent non-ALD reactions caused by mixing ofnon-adsorbed quantities of the first and second precursor gases in oneof the chambers 44, 46, 50, it is necessary for the system 10 to inhibitthe migration of Precursor 1 from first precursor zone 14 into isolationzone 20 and the migration of Precursor 2 from second precursor zone 16into isolation zone 20. Passageways 54, 56 are preferably configured torestrict the flow of gases between the zones 14, 16, 20, to avoid orlimit diffusion of precursor gases into a common zone. Passageways 54,56 may include slits sized only slightly thicker and wider than thethickness and width of the substrate passing through them, leaving onlya very small amount of headroom and margins to allow substrate 12 topass therethrough without scraping against the sides of the passageways.For example, headroom and margins may range between microns andmillimeters in certain embodiments. The passageways 54, 56 may alsoinclude elongate tunnels through which the substrate 12 passes, asdepicted in FIGS. 1, 2, and 4. Such slits and tunnels are sometimesreferred to as slit valves, although no actual moving valve gate isutilized. In some embodiments, the passageways 54, 56 include a wiperfor further restricting flow. In one such embodiment, the substrate isthreaded through opposing leaves of resilient material, such as asynthetic rubber, which wipe against opposing surfaces of the substrate.

In an alternate embodiment (not shown), the inert gas chamber 50 ofisolation zone 20 and dividers 34, 36 are eliminated, so that isolationzone 20 essentially consists of a series of long narrow passagewaysextending completely between precursor zones 14, 16. In such anembodiment, no common inert gas chamber 50 connects the passageways, soinert gas is injected directly into the passageways medially of thefirst and second precursor zones 14, 16 to help prevent precursormigration and mixing. Isolation zone 20 of this embodiment would includea manifold, or a number of manifolds, for routing inert gas lines tonozzles along the sides of the passageways. The manifold or manifoldswould be formed in the material of the reaction chamber bordering thepassageways, and may be connected to an inert gas delivery system alongthe sides of the system, rather than at an end of the system as shown inFIG. 1.

To help isolate the first precursor gas from the second precursor gas,pressure differentials are preferably established between the isolationzone 20 and the first precursor zone 14 and between the isolation zone20 and the second precursor zone 16. In one embodiment, the pressuredifferentials may be generated by injecting inert gas into isolationzone 20 at a pressure greater than the operating pressure of theprecursor zones 14, 16, and then passively exhausting gases from theprecursor zones 14, 16. In another embodiment, the exhaust fromprecursor zones 14, 16 could be controlled relative to a passive exhaustfrom isolation zone 20 or by throttling an exhaust flow from isolationzone 20. Pressure differentials may also be generated by pumping fromprecursor zones via pump 58 or another source of suction. Optionally,pump 58 may be coupled to all zones, with flow from the various zonesbeing controlled to maintain the pressure differential. The migration ofprecursors from the precursor zones 14, 16 into the isolation zone 20may also be prevented or limited by controlling both the relative flowrates of gases into the zones and pumping speeds from the zones, throughthe use of flow control valves or other flow control devices. A controlsystem (not shown) responsive to pressure sensors in the various zonesmay also be utilized to control gas injection and exhaust flow rates tohelp maintain a desired pressure differential.

In one example, isolation zone 20 operates at a pressure ofapproximately 5 millitorr (i.e., the inert gas injection pressure may be5 millitorr), and pressure differentials of approximately 0.1 millitorrare maintained between isolation zone 20 and each of the precursor zones14, 16, so that an operating pressure of approximately 4.9 millitorr ismaintained in precursor zones 14, 16 by way of suction applied toprecursor zones 14, 16 by pump 58. Lower and significantly higherpressure differentials may also be used in some embodiments. Thenecessary pressure differential will be affected by the geometry ofpassageways 54, 56 (including height, width, and tunnel length, ifapplicable), the headroom and margins around substrate 12 withinpassageways 54, 56, the transport speed of substrate 12, the surfaceroughness of substrate 12 and passageways 54, 56, and the location atwhich inert gas is injected, such as direct injection into passageways54, 56 or generally into inert gas chamber 50. Other factors, such asoperating temperature, pressure, precursor species, and substrate type,may also affect the amount of pressure differential necessary to inhibitor prevent migration of precursor gases through passageways.

In some ALD processes, precursor gases having a very low vapor pressureare utilized. To facilitate pumping and diffusion control, inert gas maybe mixed with such precursor gases, either before or after introductionof the precursor gases into the system 10, to control the pressurewithin precursor zones 14, 16.

In some embodiments, it may be desirable to equalize the pressures, orto deliberately mismatch the pressures in two or more precursor zones tooptimize growth conditions, or improve utilization of precursormaterials. It may also be desirable to pump two or more of the zonesseparately, and introduce inert gas into the precursor zones separatelyto further reduce zone migration; for instance, a cross-flow conditionmay be used to flow precursor in a direction orthogonal to thepassageways 54, 56 (between first and second ends 72, 84). Inert gas maybe introduced locally within or near passageways 54, 56, to inhibitgases from either adjacent zone from crossing through passageways 54,56. If further isolation is necessary, multiple differentially-pumpedand purged zones may be used in series, with flow-restrictingpassageways or wiper valve isolation between zones and exhaust pathsfrom each of the zones.

As described above, the precursor zones 14, 16 may be pumped to achievean isolating pressure differential between the isolation zone and theprecursor zones. In one configuration (not shown), separate pumps couldbe used for each of the zones 14, 16, 20, preventing mixing of precursorgases in the pump stack and the attendant growth of material or reactionbyproducts in any of the pumping lines, thereby preventing powder andresidue from accumulating and clogging the pump stack. Another way toinhibit undesirable material deposits in the pump stack is to trapexhaust precursors using a precursor trap 59, such as a simple inlineliquid nitrogen cooled trap, for example model TLR4XI150QF sold by KurtJ. Lesker Company (www.lesker.com). Similar precursor traps may beplaced in each of the precursor exhaust lines upstream of their junctionbefore the pump 58. By using inert gases and precursor materials havingdifferent vapor pressures at a given temperature, it may be possible totrap and reclaim up to approximately 100% of exhaust precursor gases,while passing inert gases to the pump stack. And because differentprecursors are not mixed in the zones, the precursor purity ismaintained, enabling up to 100% utilization of precursor materials. Oncefilled, traps 59 may be turned into precursor sources by replacing theliquid nitrogen with a heated liquid or by activating heating elementsoutside the trap, then reversing the pumping direction or closing anisolation valve (not shown) between pump 58 and trap 59. The particularoperating temperature of trap/source would depend on the precursor beingtrapped and its vapor pressure. A liquid nitrogen trap, for example, mayoperate at lower than 100° Kelvin. Additional trap/source configurationsare described below with reference to FIG. 4.

A substrate transport mechanism 60 of system 10 includes multipleturning guides for guiding substrate 12, including a set of firstturning guides 64 spaced apart along first precursor zone 14 and asecond set of turning guides 66 spaced apart along second precursor zone16. Turning guides 64, 66 cooperate to define an undulating transportpath of substrate 12 as it advances through system 10. The substratetransport mechanism 60 may include a payout spool 72 for paying outsubstrate 12 from a first coil (input roll 74) for receipt at a firstend 76 of isolation zone 20, vessel 30, or one of the precursor zones14, 16. The substrate transport mechanism 60 may further include atake-up spool 82 for receiving the coated substrate 12 from a second end84 of isolation zone 20, vessel 30, or one of the precursor zones 14, 16opposite first end 76, and coiling the substrate 12 into a take-up roll86 or second coil. Payout spool 72 and/or take-up spool 82 may belocated within vessel 30, such as within isolation zone 20, as depictedin FIGS. 1-2. Alternatively, payout and take-up spools 72, 82 may belocated outside of vessel 30 (i.e., outside of isolation zone 20 andfirst and second precursor zones 14, 16), as depicted in FIGS. 3 and 4.Input and take-up rolls 74, 86 will change diameter during operation ofsystem 10, and will therefore require tension control and/or drivecontrol systems of the kind well known in the art of web handling andcoil handling systems. Additional turning guides may be provided fordetermining the transport path of substrate 12 through, and in someembodiments, into, the vessel 30. For example, additional turning guides(not shown) may be required to compensate for changes in the diameter ofthe input and take-up rolls 74, 86 during operation of system 10.

Turning guides 64, 66 may comprise rotating guide supports, such asrollers, pulleys, sprockets, or pinch rollers, as well as non-rotatingguide supports, such as guide bars, rails, or channels. Suitablerotating guide supports include both idlers, e.g. idler rollers, anddriven rotating supports—the latter being driven by a drive mechanism(not shown) that may include means for synchronizing the rotating guidesupports with each other and with payout spool 72 and/or take-up spool82. Non-rotating guide supports may preferably include a bearing surfacemade of or coated with a low-friction material, such as PTFE (TEFLON™).In one embodiment, turning guides 64, 66 may comprise fluid bearings(e.g. gas bearings) that support substrate 12 on a dynamic cushion offluid, such as precursor gas and/or inert gas injected through smallperforations in a bearing race of the fluid bearing.

Depending on the configuration of substrate transport mechanism 60 andpassageways 54, 56, the transport path of substrate 12 may have aserpentine profile, a sawtooth profile, or any other suitable shape fortransporting substrate between first and second precursor zones 14, 16.Substrate 12 preferably threads through passageways 54, 56 and traversesisolation zone 20 in a direction normal to the plane of dividers 32, 34,such that opposing pairs of first and second passageways 54, 56 arealigned with a traversal axis normal to dividers 32, 34. However, otherarrangements and transport path configurations may also be utilized.

In the embodiment shown, each of the first turning guides 64 ispositioned within the first precursor zone 14 and supports substrate 12as it turns 180° about the turning guide 64 toward the second precursorzone 16. Similarly, each of the second turning guides 66 is positionedwithin the second precursor zone 16 and supports substrate 12 as itturns 180° about the turning guide 66 toward the first precursor zone14. In an alternative embodiment (not shown), only some of the turningguides 64, 66 may support substrate 12 as it turns toward the oppositeprecursor zone. For example, two turning guides may be used for a single180° turn, each supporting the substrate through 90° of the turn. Inother embodiments, substrate 12 may turn through somewhat more or lessthan 180° between traversals of isolation zone 20. A turn of greaterthan 180° could be implemented to fit more turning guides, and thereforemore deposition cycles, within a system of a given overall length. Atransit path of substrate 12 through precursor zones 14, 16 may becurved and/or straight. In one embodiment (not shown), some or all ofthe first and second turning guides may be located outboard of therespective first and second precursor zones such that the substratefollows a straight transit path completely across the respectiveprecursor zone nearest the turning guide and through passageways individers bordering the inboard and outboard sides of the respectiveprecursor zone.

The system 10 illustrated in FIG. 1 includes ten first turning guides 64and ten second turning guides 66, providing ten full cycles of ALDgrowth. In one example, the system of FIG. 1 may be used to deposit acoating of aluminum oxide (Al₂O₃) approximately ten angstroms (10 Å)thick using trimethylaluminum (TMA) as Precursor 1 and water asPrecursor 2. Additional ALD cycles may be added to system 10 by addingpairs of turning guides. For example, a 100-cycle system may have 200turning guides—100 first turning guides 64 and 100 second turning guides66. By using small-diameter guide rollers or other turning guides, sucha system could be as small as one meter long from the input roll 74 tothe take-up roll 86, approximately 50 cm high, and only slightly widerthan the width of substrate 12. Systems capable of 500, 1000, 5000, ormore ALD cycles in a single pass are also envisioned. Similar expansionsare possible in the systems of FIGS. 2 and 4, described below.

To increase film thickness beyond what is deposited in a single passthrough system 10 by the number of ALD cycles defined by transportmechanism 60, the substrate 10 may be passed through the system multipletimes, either by moving the take-up roll 86 from the second end 84 tothe first end 76 after a pass, by reversing the transport direction ofthe substrate 12 to send it back through the system, or by using aclosed-loop substrate that circulates back to the input side 76 forachieving multiple passes through the system without movement orhandling of the bulk roll. In between sequential passes, one or more ofthe precursors within the precursor zones 14, 16 may be changed toprovide a multi-layer coating stack of two or more thin film materials.

FIG. 2 illustrates a system 110 and method according to a secondembodiment for depositing layers of different materials on a flexiblesubstrate 112 in a single pass through system 110. In the embodiment ofFIG. 2, multiple separate precursor zones are located in sequence alongthe length of the reaction chamber. In FIG. 2, 100-series referencenumerals with the last two digits similar to the reference numerals ofFIG. 1 designate similar components. For example, system 110 includes afirst precursor zone 114 supplied by a first precursor delivery system124, a second precursor zone 116 supplied by a second precursor deliverysystem 126, and an isolation zone 120 supplied by an inert gas deliverysystem 128. System 110 of FIG. 2 further includes a third precursor zone190 into which a third precursor gas (Precursor 3) different from thefirst and second precursor gases (Precursor 1 and Precursor 2) isintroduced when the system is in use. Third precursor zone 190 isseparated from isolation zone 120 by a third divider and positionedopposite second precursor zone 116. In the embodiment shown, the thirddivider is a middle section of upper divider 134, which includes aseries of third passageways 192 therethrough, spaced apart along thirdprecursor zone 190. Similarly, a fourth precursor zone 194 for receivinga fourth precursor gas (Precursor 4) is positioned opposite secondprecursor zone 116 and separated from isolation zone 120 by an endsection of upper divider 134, through which a series of spaced-apartfourth passageways 196 are provided. Precursor 4 is preferably differentfrom Precursor 1, Precursor 2, and Precursor 3, but may alternatively bethe same as Precursor 1 to achieve deposition of alternating layers ofthin film materials. Third precursor zone 190 is isolated from first andfourth precursor zones 114, 194 by a pair of partition walls 198 atopposite ends of third precursor zone 190, each extending between upperdivider 134 and an outer reaction chamber wall 132 of vessel 30.

In the embodiment of FIG. 2, more than two precursor zones are utilizedto fabricate multiple layers of distinct materials—for example a firstten serpentine paths may traverse between first precursor zone 114 andsecond precursor zone 116, respectively, and the next ten serpentinepaths may traverse between third precursor zone 190 and second precursorzone 116, finally, etc., resulting in multi-layer film stacks.

In one example, the system 110 illustrated in FIG. 2 may utilize TMA asPrecursor 1, water as Precursor 2, TiCl₄ as Precursor 3, and TMA asPrecursor 4 to coat 3 cycles of Al₂O₃ (approximately 3 Å), followed by 4cycles of titania (TiO₂) (approximately 2 Å), followed by another 3cycles of Al₂O₃.

In another example, a thin film of aluminum-doped zinc oxide (ZnO) maybe formed utilizing a system similar to the one shown in FIG. 2.Aluminum-doped ZnO is an optically transmissive conductive oxide filmthat may be useful as a substitute for more expensive indium-tin-oxide(ITO) electrodes commonly used in electronics and solar cells. In thisexample, diethylzinc (DEZn) or dimethylzinc (DMZn) are used as Precursor1 and Precursor 4, and each of the first and fourth precursor zones 114,194 includes between 50 and 100 turning guides (i.e., the substratetransits between 50 and 100 times in each of the first and fourthprecursor zones). An oxidant, such as water, or more preferably ozone,is used as Precursor 2, and TMA is used as Precursor 3. The thirdprecursor zone 190 may include only a very small number of turningguides (and transits)—for example two—to deposit only a doping amount ofAluminum oxide within the bulk ZnO. The substrate may then betransported through the system multiple times, in multiple passes, toachieve the desired mechanical, electrical, and optical properties.

In an alternative embodiment (not illustrated), third precursor zone 190may be positioned between first and second precursor zones 114, 116 sothat isolation zone 120 straddles third precursor zone 190 and substrate112 traverses across third precursor zone 190 as it is transportedbetween first and second precursor zones 114, 116. Other variations onthe configuration of system 110 are also possible, the variety ofconfigurations preferably having their various precursor zones isolatedfrom each other by one or more isolation zones, to prevent precursorgases from reacting in any of the zones, except at the surface ofsubstrate 112.

An alternative system 200 shown in FIG. 3 may be configured withoutrollers, yet achieve ALD-type deposition on a long thin substrate 212,such as a web, by passing the substrate 212 along a linear transportpath between alternating zones 202, 204, 206, etc., of precursor 1,inert gas, precursor 2, inert gas, precursor 1, inert gas, etc. In FIG.3, exhaust or pumping lines from precursor zones 202, 206, etc. areomitted for simplicity. While system 200 would likely be much longerthan those of FIGS. 1 and 2 for a given layer count, the system 200 ofFIG. 3 could be made very thin, for example if configured as astraight-line system such as ones used for architectural glass coatingsystems. Accordingly, system 200 could be used to coat both flexiblesubstrates and rigid substrates. It could also reduce issues arising, inthe systems 10 and 110 of FIGS. 1 and 2, from contact between substrate12 and the turn guides 64, 66 of substrate transport mechanism 60. Inone embodiment, precursor 1 is TMA and precursor 2 is water vapor, andone pass of the substrate 212 through the system completes three ALDcycles to deposit approximately three angstroms (3 Å) of aluminum oxide(Al₂O₃). One variation on the configuration of FIG. 3 would be to have achamber with as few zones as four, e.g., precursor 1, inert gasisolation, precursor 2, and inert gas isolation, to provide one full ALDcycle. A closed-loop substrate of flexible material (not shown) could becirculated through such a system, and the number of trips orcirculations of the loop substrate through the chamber would determinethe resulting coating thickness.

Some systems and methods of the kind described herein may notnecessarily require highly specific geometry or mechanicalconfiguration. For instance, in addition to the configurationsillustrated in FIGS. 1-3, the substrate could be wound through a paththat looks like a “zig-zag” or a sine wave, or any path, as long as thesubstrate winds sequentially through regions that provide at least thefollowing: (1) exposure to one precursor; (2) an isolation zone, whereinthe substrate is not exposed to one of the primary precursors; (3)exposure at least a second precursor; and (4) a second isolation zone asin step (2), which may be a common zone as that used for step (2). Thesubstrate does not necessarily have to pass over rollers—essentially anymechanical arrangement that allows the traversal or threading of thesubstrate through the sequential zones would work.

FIG. 4 illustrates a system 310 according to a fourth embodiment,wherein the last two digits of 300-series reference numerals designatingprecursor zones 314, 316, isolation zone 320, and components ofsubstrate transport mechanism 360 correspond to similar 2-digitreference numerals identifying similar elements in the embodiment ofFIG. 1. With reference to FIG. 4, system 310 includes input and take-uprolls 374, 386 located outside of the reaction chamber housing 330.Additional input/output turning guides 338 are provided within isolationzone 320. Substrate 312 is fed through one or more slits, wiper valves,or other flow-constricting entrance and/or exit passageways 340, 342.Positioning the input and take-up rolls 374, 386 outside of reactionchamber 330 may ease roll loading and unloading.

In an alternative embodiment (not shown), the input and take-up rolls374, 386 may be placed in separate vacuum chambers or load-locksadjacent the first and second ends 376, 384 of reactor housing 330.Additional process modules may be provided between input roll 374 andreaction chamber 330 and/or between reaction chamber 330 and take-uproll 386 such that the thin film coating process would comprise just onemodule in a larger substrate processing system. For example, apreheating stage or other functionalization module may be providedbetween input roll 374 and reaction chamber 330. Examples ofpre-treating or functionalizing steps useful with ALD coating system 310include vacuum processing to speed up out-gassing of substrate 312before coating; ultra-violet light treatment; ozone treatment, e.g., formaking normally-hydrophobic plastic films hydrophilic to enhance orenable ALD processing; exposure to a plasma or other source of radicals;and cleaning steps. Other process modules, such as lithography and otherpatterning steps, non-ALD deposition such as sputtering, and othersurface finishing and coating steps, may also be utilized.

System 310 includes adjustable turning guides 364, 366 that are movabletoward and away from dividers 334, 336 and isolation zone 320 to changea substrate dwell time within the precursor zones 314, 316. The locationof turning guides 364, 366 may be adjusted independently or in groups,and may be controlled by a control system 310 to change dwell time asprocess needs change over time. In FIG. 4, three different groups ofturning guides are shown in each precursor zone, each group having adifferent dwell time. Adjusting dwell time may facilitate nucleation forcertain precursors, and may improve precursor penetration in poroussurfaces. Similarly, dividers 334, 336 may be movable along the samedirection as adjustable turning guides 364, 366 (i.e., up and down), tochange the substrate dwell time in isolation zone 320.

System 310 further includes a precursor recycling subsystem 400 locatedupstream of a junction 404 of the pumping/exhaust lines from first andsecond precursor zones 314, 316. Subsystem 400 includes first and secondtraps 410, 420 flanked by a pair of three-way valves 430, 432, or theirequivalent, to enable one of the traps 410, 420 to be selectivelyinterposed in the pumping line 440 between the precursor zone 316 andpump 358. A first one of the valves 430 includes two outlets, one beingconnected to the inlet of first trap 410 and the other connected to theinlet of second trap 420. Similarly, second valve 432 includes twoinlets; one connected to the outlet of first trap 410 and the other tothe outlet of second trap 420. FIG. 4 shows the left-hand sides ofvalves 430, 432 being closed and the right-hand sides open, so thatsecond trap 420 is interposed in the pumping line 440 and serving as aprecursor trap. Meanwhile, first trap 410 is isolated from the pumpingline 440, as indicated by blackened shut-off valves on the left-handside of each three-way valve 430, 432. First trap 410 is operating in aregenerative mode, whereby the trap 410 is being heated to releasepreviously trapped precursor material into a precursor supply/recyclingline 450. Isolation valves 462, 464 are provided between supply outletsof respective traps 410, 420 and a downstream supply junction 470 of thesupply outlets in supply/recycling line 450. The position of valves 430,432, 462, and 464 may be reversed from what is shown in FIG. 4, so thatfirst trap 410 functions as the inline precursor trap, and second trap420 operates as a precursor source. In the embodiment shown, one half ofsubsystem 400 is always operating as a trap, and the other half as asupply component of precursor delivery system 326.

A second subsystem (not shown) may be provided in the first precursorzone exhaust line upstream of junction 404 for trapping and recyclingfirst precursor in a similar manner.

Suitable traps 410, 420 for subsystem 400 may include simple inlineliquid nitrogen traps or, more preferably, cryogenic “waterpumps”modified to pump a precursor, rather than water, at suitable pressuresand temperatures. Suitable cryogenic waterpumps preferably includebuilt-in heaters for regeneration capability. Example of cryogenicwaterpumps include the Polycoldo PFC water vapor cryopump and theCTI-Cryogenics® LowProfile Waterpump™, both sold by Brooks Automation(www.brooks.com). Cryogenic waterpumps are normally configured to pumpin a low vacuum environment, but may be modified or adjusted to work inthe operating pressure ranges of methods described herein. For someprecursors the trap operating temperature may range from 100-150°Kelvin, while for others, it may range between 1500 and 3000 Kelvin.Higher trapping temperatures may enable certain metal halide precursorchemicals to be trapped, while allowing other materials, such asbackground water vapor, solvents, and inert gas, to pass, therebyimproving the purity of trapped precursor.

The systems and methods described herein may exhibit little or nocoating of moving machine parts, including the payout and take-up spools72, 82, 172, 182, 272, 282, 372, 382 and turning guides 64, 66, 164,166, 364, 366, since each of these parts is either resident in only onezone of the system, or entirely outside of the zones. Unlikeconventional ALD systems, high-speed pulse valves are not required inthe systems described herein and, in theory, maintenance requirementswould be minimal.

Systems and methods consistent with the embodiments disclosed herein mayoperate over a relatively wide range of temperatures and pressures.Necessary operating temperatures and pressures will largely bedetermined by specific process chemistry. However, for example,operating pressures may range from relatively low vacuum environments ofapproximately 1 millitorr, to positive pressure environments of 500-1500Torr (approximately 1 to 2 atmospheres). Pressures may be different indifferent precursor zones, to accommodate the use of precursors havingdifferent vapor pressure, molecular mobility, and reactivitycharacteristics, for example. In some embodiments, two or more precursorzones and the inert gas zone may be maintained at different temperaturesto optimize film properties and/or throughput. Operating temperaturesmay also vary from below room temperature to well above roomtemperature, at operating temperatures typical of traveling wave ALDreactors.

Heated rollers or turning guides 64, 66, 164, 166, 364, 366 may beutilized in some embodiments, to heat the substrate and promote thinfilm growth via ALD. One or more of the precursor zones 14, 16, 114,116, 314, 316 and/or the isolation zone 20, 120, 320 may also be heated.The passageways 54, 56, 154, 156, 354, 356 may be heated by injecting aheated inert gas directly into the passageways.

In one embodiment, a plasma discharge or other source of radicals isincluded in one or more of the precursor zones, or in an adjacentchamber, to enable plasma- or radical-assisted ALD film growth.

The systems and methods described herein will normally result indeposition on both faces of the substrate. To achieve single-sideddeposition, the substrate may be layered, folded, or masked fordeposition, then peeled apart, unfolded, or the mask removed to resultin a finished product. Other possible methods of single-sided depositioninclude deposition on a flattened tubular substrate followed by slittinglengthwise, or slitting of a solid substrate after double-sideddeposition.

The systems and methods described herein are not limited to depositionon web substrates such as plastic films or metal foil. The same basicconfiguration could be used to coat wire, flexible tubing, wovenmaterials, such as cloth, braided materials such as braided wire orrope, non-woven sheet materials such as paper, construction vaporbarrier, etc.

The following are further examples of potential applications for thesystems and methods disclosed herein:

1) On plastic or metal foil, as a gas or chemical barrier, as anelectrical insulator, as an electrical conductor, or as a semiconductor.Specific applications include oxygen and moisture barriers for food ormedical packaging, electrically insulating, conducting orsemi-conducting films for large area solar cells, flexible displays, andflexible electronics.2) Coatings on woven materials such as cloth to provide fireretardation, or functionalize the surface—to provide moisture or stainresistance, for example.3) Gas or chemical barriers or tubing, such as plastic tubing used inchemical or medical applications.4) Mechanical/physical property improvements in woven or pressed-sheetmaterials—for example, a film that could provide “filler” to join andbind the individual particles or fibers.

Throughout this specification, reference to “one embodiment,” or “anembodiment,” or “some embodiments” means that a particular describedfeature, structure, or characteristic is included in at least oneembodiment. Thus appearances of the phrases “in one embodiment,” “in anembodiment,” “in some embodiments,” and the like, in various placesthroughout this specification are not necessarily all referring to thesame embodiment. Furthermore, the described features, structures, orcharacteristics may be combined in any suitable manner in one or moreembodiments. In some cases, the invention may be practiced without oneor more of the specific details or with other methods, components,materials, etc. In other instances, well-known structures, materials, oroperations are not shown or not described in detail to avoid obscuringaspects of the embodiments.

It will be obvious to those having skill in the art that many changesmay be made to the details of the above-described embodiments withoutdeparting from the underlying principles of the invention. The scope ofthe present invention should, therefore, be determined only by thefollowing claims.

The invention claimed is:
 1. A system for depositing a thin film on aflexible substrate, comprising: a first precursor zone into which afirst precursor gas is introduced when the system is in use; a secondprecursor zone into which a second precursor gas different from thefirst precursor gas is introduced when the system is in use; anisolation zone interposed between the first and second precursor zonesand into which an inert gas is introduced when the system is in use, theisolation zone including a series of flow-restricting passagewaysopening into the first and second precursor zones and arranged toprovide a series of pathways along which a flexible substrate isthreaded along a path that traverses back and forth between the firstprecursor zone and the second precursor zone multiple times and eachtime through the isolation zone; and a substrate transport mechanism,including: a plurality of first turning guides spaced apart along thefirst precursor zone, at least some of the first turning guides adaptedto support the substrate during a change in a direction of travel of thesubstrate toward the second precursor zone, and a plurality of secondturning guides spaced apart along the second precursor zone, at leastsome of the second turning guides adapted to support the substrateduring a change in a direction of travel of the substrate toward thefirst precursor zone.
 2. The system of claim 1, in which at least someof the passageways include a tunnel.
 3. The system of claim 1, in whichat least some of the passageways include a wiper.
 4. The system of claim1, further comprising a first divider separating the isolation zone fromthe first precursor zone, and a second divider separating the isolationzone from the second precursor zone, and in which the flow-restrictingpassageways includes a series of first passageways through the firstdivider and a series of second passageways through the second divider.5. The system of claim 1, wherein the path includes a serpentine paththat traverses back and forth between the first precursor zone and thesecond precursor zone multiple times and each time through the isolationzone.
 6. The system of claim 1, in which each of the first and secondturning guides includes a roller.
 7. The system of claim 1, in which thesubstrate transport mechanism further comprises: a payout spool forpaying out the substrate from a coil for receipt at a first end of theisolation zone; and a take-up spool for coiling the substrate receivedfrom a second end of the isolation zone opposite the first end.
 8. Thesystem of claim 7, in which the payout and take-up spools are locatedwithin the isolation zone.
 9. The system of claim 7, in which the payoutand take-up spools are located outside the isolation zone, the firstprecursor zone, and the second precursor zone, and further comprising aninlet passageway at a first end of the isolation zone and an outletpassageway at a second end of the isolation zone opposite the first end.10. The system of claim 7, in which the payout spool and the take-upspool each includes a reversible drive mechanism.
 11. The system ofclaim 1, further comprising: a first precursor delivery system coupledto the first precursor zone for supplying the first precursor gas to thefirst precursor zone; and a second precursor delivery system coupled tothe second precursor zone for supplying the second precursor gas to thesecond precursor zone.
 12. The system of claim 1, further comprising aninert gas delivery system for injecting an inert gas into the isolationzone.
 13. The system of claim 1, in which the series of flow-restrictingpassageways includes at least ten such passageways.
 14. The system ofclaim 1, in which at least some of the first turning guides areadjustably mounted within the first precursor zone for movement towardand away from the isolation zone, to thereby adjust a dwell time of thesubstrate in the first precursor zone.
 15. The system of claim 1,further comprising a pump operably coupled to at least one of the firstand second precursor zones.
 16. The system of claim 15, furthercomprising a precursor trap interposed between one of the first andsecond precursor zones and the pump.
 17. The system of claim 1, furthercomprising a radicals generator for supplying a precursor radical to oneof the first and second precursor zones.
 18. The system of claim 1,further comprising: a third precursor zone separated from the first andsecond precursor zones and into which a third precursor gas differentfrom the first and second precursor gases is introduced when the systemis in use; a third divider separating the third precursor zone from theisolation zone; and a series of third passageways extending through thethird divider and into the third precursor zone from the isolation zone,the third passageways spaced apart along the third precursor zone andarranged for the substrate to be threaded therethrough back and forthbetween the second and third precursor zones and through the isolationzone.
 19. The system of claim 1, in which at least some of the turningguides include a heated roller.
 20. The system of claim 1, furthercomprising: an exhaust line connected to one or more of the precursorzones; and a precursor trap located in the exhaust line.
 21. The systemof claim 20, further comprising a recycling line connected between theprecursor trap and one or more of the precursor zones.
 22. The system ofclaim 4, wherein the path includes a serpentine path that traverses backand forth between the first precursor zone and the second precursor zonemultiple times and each time through the isolation zone.